This invention relates to a process for the treatment of wastes in the form of liquids or aqueous sludges, e.g., industrial, agricultural and domestic wastes. More particularly, this invention relates to a process for the solidification of wastes to convert the latter into solid materials having great stability with respect to external agents, such as water, and a high retention power for the pollutant materials contained in these wastes. In another aspect, this invention relates to the use for various purposes of the thus-treated waste material.
The growing volume of wastes and residues of all types, in the form of liquids, sludges and slurries, presents greater and greater problems for manufacturers and municipalities, particularly due to the fact that the waste materials present serious hazards to the environment and living beings. In some cases, it is possible to incinerate the sludges if they have sufficiently high fuel content. In other cases, they can be spread on agricultural or wooded lands. However, very often the only solution acceptable from an economic and ecological point of view is to treat the sludges in order to achieve fixation of the pollutants therein by physico-chemical solidification.
For this purpose, methods have been proposed which consist of mixing the sludges to be treated with appropriate proportions of various products such as, for example, Portland cement alone or together with pozzolana (finely divided siliceous, or siliceous and aluminous material which reacts chemically with slaked lime at ordinary temperatures in the presence of moisture to form a strong, slow-hardening cement), sodium silicate together with a setting agent, such as lime, calcium chloride, Portland cement, calcium carbonate, mixtures of lime and pozzolana, gypsum or plaster of Paris, optionally together with organic resins, bitumens, calcium sulfate and sulfite mixed with lime and/or possolana.
These various methods provide solutions in a few special cases but they have serious drawbacks which prevent their general use. Moreover, the treated sludges sometimes have a very basic pH which can be higher than 10 and comprise a relatively high content of leachable ions and molecules noxious to the environment, for example, Na.sup.+, Cu.sup.2+, Zn.sup.2+, Ca.sup.2+, Cr.sup.2+, SO.sub.4.sup.2-, F.sup.-, PO.sub.4.sup.3-, and hydrocarbons.
The pollutant components of the thus-treated waste, theoretically fixed in the solid mass produced by such a treatment, actually are partially removable therefrom, e.g., by water lixiviation (leaching), by the action of atmospheric agents, e.g., hot air, or as the result of cyclical freezing and thawing. It appears that the abnormally high content of some non-fixed ions comes from one or several of the reactants used in some of these methods. For example, non-fixed sodium contents higher than attributable to the lixivation of the raw waste material, all the other conditions being the same, can be explained by the use of sodium silicate as the main reactant. These known methods also are costly because the quantitites of reactants used are high and the reactants generally employed, e.g., Portland cement, sodium silicate, etc., are expensive. Most of the known methods are selective for a given type of sludge and, in some cases, they give rise to the formation of water-soluble free lime which plays a significant role in the phenomenon which renders the structure of thus-produced metal silicate fragile.
It is an object of this invention to obviate these drawbacks of the prior art processes by providing a process useful for the treatment of a wide variety of sludges and aqueous wastes to produce solidified and hardened composite materials having high inertness and stability under usual ecological conditions. In particular, this invention converts the pollutants in wastes into solid products which, when contacted with water, for example, do not liberate a large part of the pollutant compounds fixed therein.